Essential Elements for Plant Growth

All 90 or so naturally-occurring
elements are found in normal plant tissue. Only 16 or so elements
are truly essential for plant growth. The rest of the elements present in
plant tissue are largely taken up in small quantities incidentally (or
accidentally!) as plants take up the nutrient elements that they need for
growth and reproduction.

Although common sense goes a long way in defining the concept of an
essential element, a more precise set of criteria were established
by Arnon and Stout in 1939, who stated that an essential element:

Must required for the completion of the life cycle of the plant.

Must not be replaceable by another element.

Must be directly involved in plant metabolism, that is, it must be
required for a specific physiological function.

To Arnon and Stout's three requirements for essential elements should be
added a fourth:

The element must required by a substantial number of plant species,
not just a single species or two.

It is surprisingly difficult to prove that
small quantities of an element are essential for plant growth, even in
experiments using hydroponics. Seeds will
contain a finite concentration of the element being tested, and parent
plants will pass on the element tested from generation to generation.
Chemical reagents used to prepare liquid fertilizers may be 99.99% pure,
but the impurities in the reagents, in water, and even leaching out of the
pots themselves make it difficult to reach absolute zero concentrations of
the element being tested.

Although protein chemistry has made many advances, determining the
exact physiological function of elements has lagged behind proof of the
essential nature of some elements.

Additional ambiguities stretch Arnon's
criteria for essential elements in interesting ways, but the list of
essential elements is not likely to be reduced in size, and may grow
slowly.